add bgm playback, scrolling based on bgm time

Scripts/Scenes/Play/Audio/BGM.cs

@@ -0,0 +1,46 @@
+using Godot;
+using Godot.Collections;
+using System;
+
+public partial class BGM : AudioStreamPlayer
+{
+	public void LoadFromUser(string path)
+	{
+		if (!path.StartsWith("user://"))
+		{
+			GD.Print("Tried to load audio that isn't in user directory.");
+			return;
+		}
+
+		var f = FileAccess.Open(path, FileAccess.ModeFlags.Read);
+		if (f == null)
+		{
+			GD.PrintErr($"Unable to open {path} for loading audio! {FileAccess.GetOpenError()}");
+			return;
+		}
+		GD.Print("hi");
+		var ext = path.Split('.')[^1].ToLower();
+		switch (ext)
+		{
+			case "mp3":
+				var mp3 = new AudioStreamMP3()
+				{
+					Data = f.GetBuffer((long)f.GetLength())
+				};
+				Stream = mp3;
+				break;
+			case "wav":
+			case "wave":
+				var wav = new AudioStreamWav()
+				{
+					Data = f.GetBuffer((long)f.GetLength())
+				};
+				Stream = wav;
+				break;
+			case "ogg":
+				// TODO: implement
+				GD.PrintErr("External OGGs not supported in Godot 4.1...");
+				break;
+		}
+	}
+}

Scripts/Scenes/Play/Audio/SFX.cs

@@ -0,0 +1,15 @@
+using Godot;
+using System;
+
+public partial class SFX : Node
+{
+	// Called when the node enters the scene tree for the first time.
+	public override void _Ready()
+	{
+	}
+
+	// Called every frame. 'delta' is the elapsed time since the previous frame.
+	public override void _Process(double delta)
+	{
+	}
+}